add "linear" size_function example to the tutorial

[grahamgower]

See popsim-consortium/demes-python#296 and popsim-consortium/demes-python#310

We probably want an example in the tutorial as well as a description in the spec. In the spec, we should explicitly state that when the size_function isn't specified, it is set to "constant" (if start_size == end_size) or "exponential" (if start_size != end_size).

[grahamgower]

There's a separate issue for defining the size_function defaults in the spec (#34), so we probably just need a tutorial entry for this.